Lithographic printing tools (e.g. scanners, steppers, etc. and referred to henceforth as “a scanner”) need to maintain their focus and dose values within some node-defined process window. Consequently, there are strict requirements on both focus and dose as scanner parameters. For recent nodes, the allowed range of focus variations is +10 nm around its nominal position and this requirement will become tougher for the next nodes. A general approach for determining the actual scanner's focus position consists of printing of a special target, which is sensitive to focus changes of the scanner on product wafers and measuring some corresponding property (or properties) of the printed pattern. The dose is measured in the same way. The relation between the measured properties and the scanner focus and/or dose is established using a special test wafer (FEM—focus exposure matrix wafer), in which the same pattern is printed for different predefined scanner focus and dose values.
Currently, models are generated and calibrated on the basis of standard FEM, in which predefined focus positions change along one direction (e.g., X) while predefined dose values change along the perpendicular direction (e.g., Y). Hence, successful model creation, which is based on FEM wafer usage together with designing of targets having high sensitivity to scanner focus changes, is the most important component of focus and dose measurement approach. However, since the scanner focus errors due to process variations on product wafer are about 20 nm, the same focus errors should be expected on FEM wafers. Such a large ambiguity in scanner focus positions on FEM wafers does not allow the generation of sufficiently finely calibrated models and may cause large errors in subsequent focus/dose measurements. In order to reduce correlation between focus/dose characteristics and process variations, a random FEM is used. In this case, the sites corresponding to different values of focus and dose are randomly placed in a wafer area.